The aim of my proposal is to examine the in vivo functions and the mechanisms of action of novel inhibitory costimulatory molecules PD-PDL and B7-H3 in allograft rejection with the goal of harnessing their function to achieve allograft tolerance. This topic has great clinical relevance for transplantation in general as blockade of conventional positive T cell costimulatory pathway such as B7-CD28 has not been effective in reproducibly inducing tolerance in stringent murine transplant models as well as in larger animals such as primates. An important theme of my proposal is that harnessing physiologic mechanisms of regulation by novel negative T cell costimulatory pathways in addition to positive T cell costimulatory blockade are critical to achieve tolerance in humans. I have generated extensive data about the role of negative costimulatory pathways CTLA4-B7 and PD-1/PDL in CD4 and CDS mediated alloimmune responses in vivo. I also have promising preliminary data about the role of the recently described member of CD28 family, B7-H3 in a vascularized cardiac model. I plan to extend these studies using a number of unique tools that will enable me to dissect the functions, mechanisms and interactions of negative and positive costimulatory pathways in regulating alloimmune responses in vivo. These include well established animal models, novel gene knockout animals targeting specific costimulatory molecules, agents to target novel costimulatory pathways and finally elegant models of CD8+ and CD4+ TCR transgenic mice that allow tracking of alloreactive T cell responses in vivo. The Mentored Clinical Scientist Development Award will provide me with the critical opportunity to further expand my knowledge in immunology necessary for achieving independence. My mentor's and co-mentor's laboratories provide an ideal environment to accomplish the aims of my project because of their strong background in transplantation immonology, their collaborative momentum and their exemplary didactic and ethical curricula.